Fix leaking MAC labels for IPv6 inpcbs by adding missing MAC label

destroy call; this transpired because the inpcb alloc path for IPv4/IPv6
is the same code, but IPv6 has a separate free path.  The results was
that as new IPv6 TCP connections were created, kernel memory would
gradually leak.

MFC after:	3 days
Reported by:	tanyong <tanyong at ercist dot iscas dot ac dot cn>,
		zhouzhouyi

sys/netinet6/in6_pcb.c

@@ -66,6 +66,7 @@ __FBSDID("$FreeBSD$");
 #include "opt_inet.h"
 #include "opt_inet6.h"
 #include "opt_ipsec.h"
+#include "opt_mac.h"
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -106,6 +107,8 @@ __FBSDID("$FreeBSD$");
 #include <netipsec/key.h>
 #endif /* IPSEC */
 
+#include <security/mac/mac_framework.h>
+
 struct	in6_addr zeroin6_addr;
 
 int
@@ -434,6 +437,9 @@ in6_pcbfree(struct inpcb *inp)
 	if (inp->inp_moptions != NULL)
 		inp_freemoptions(inp->inp_moptions);
 	inp->inp_vflag = 0;
+#ifdef MAC
+	mac_inpcb_destroy(inp);
+#endif
 	INP_UNLOCK(inp);
 	uma_zfree(ipi->ipi_zone, inp);
 }